Linux PCI driver 작성법

소개와 참고 자료

저자는 Martin Mares와 Grant Grundler입니다. PCI 세계는 방대하고 대부분 불쾌한 surprise로 가득합니다. CPU architecture마다 chipset 구현이 다르고 PCI device마다 서로 다른 요구, 즉 feature가 있어 Linux kernel의 PCI 지원은 기대만큼 단순하지 않습니다.

이 문서는 PCI device driver 작성자에게 Linux PCI API를 소개합니다. 더 완전한 자료는 Jonathan Corbet, Alessandro Rubini, Greg Kroah-Hartman의 『Linux Device Drivers』 3판이며 Creative Commons license로 `https://lwn.net/Kernel/LDD3/`에서 무료로 볼 수 있습니다.

모든 문서는 시간이 지나면 낡을 수 있으므로 설명대로 동작하지 않으면 source code를 확인하십시오. Linux PCI API 질문·의견·patch는 `Linux PCI <linux-pci@atrey.karlin.mff.cuni.cz>` mailing list로 보내십시오.

.. SPDX-License-Identifier: GPL-2.0

==============================
How To Write Linux PCI Drivers
==============================

:Authors: - Martin Mares <mj@ucw.cz>
          - Grant Grundler <grundler@parisc-linux.org>

The world of PCI is vast and full of (mostly unpleasant) surprises.
Since each CPU architecture implements different chip-sets and PCI devices
have different requirements (erm, "features"), the result is the PCI support
in the Linux kernel is not as trivial as one would wish. This short paper
tries to introduce all potential driver authors to Linux APIs for
PCI device drivers.

A more complete resource is the third edition of "Linux Device Drivers"
by Jonathan Corbet, Alessandro Rubini, and Greg Kroah-Hartman.
LDD3 is available for free (under Creative Commons License) from:
https://lwn.net/Kernel/LDD3/.

However, keep in mind that all documents are subject to "bit rot".
Refer to the source code if things are not working as described here.

Please send questions/comments/patches about Linux PCI API to the
"Linux PCI" <linux-pci@atrey.karlin.mff.cuni.cz> mailing list.

PCI driver 구조와 생명주기

PCI driver는 `pci_register_driver()`로 system의 PCI device를 발견한다고 표현하지만 실제로는 generic PCI code가 새 device를 발견하고 일치하는 description을 가진 driver에 알립니다.

`pci_register_driver()`는 device probing 대부분을 PCI layer에 맡기며 hot-pluggable PCI, CardBus, ExpressCard의 online insertion/removal을 한 driver에서 지원합니다. Function pointer table을 전달하므로 driver의 상위 구조도 결정합니다.

Device를 소유한 driver는 일반적으로 device enable, MMIO/IOP resource 요청, streaming·coherent DMA mask 설정, shared control data 할당·초기화, 필요 시 config space 접근, IRQ handler 등록, chip의 LAN/SCSI 등 non-PCI 부분 초기화, DMA/processing engine enable 순으로 진행합니다.

Device 사용을 끝낼 때는 IRQ 생성 disable, IRQ 해제, DMA 중지, streaming·coherent buffer 해제, 다른 subsystem 등록 해제, MMIO/IOP resource 해제, device disable 순으로 정리합니다.

나머지 세부 사항은 LDD3 또는 `<linux/pci.h>`를 참조하십시오. `CONFIG_PCI`가 설정되지 않으면 아래 PCI function 대부분은 driver의 많은 `#ifdef`를 피하도록 빈 inline function 또는 적절한 error를 반환하는 inline function으로 정의됩니다.

Structure of PCI drivers
========================
PCI drivers "discover" PCI devices in a system via pci_register_driver().
Actually, it's the other way around. When the PCI generic code discovers
a new device, the driver with a matching "description" will be notified.
Details on this below.

pci_register_driver() leaves most of the probing for devices to
the PCI layer and supports online insertion/removal of devices [thus
supporting hot-pluggable PCI, CardBus, and Express-Card in a single driver].
pci_register_driver() call requires passing in a table of function
pointers and thus dictates the high level structure of a driver.

Once the driver knows about a PCI device and takes ownership, the
driver generally needs to perform the following initialization:

  - Enable the device
  - Request MMIO/IOP resources
  - Set the DMA mask size (for both coherent and streaming DMA)
  - Allocate and initialize shared control data (pci_allocate_coherent())
  - Access device configuration space (if needed)
  - Register IRQ handler (request_irq())
  - Initialize non-PCI (i.e. LAN/SCSI/etc parts of the chip)
  - Enable DMA/processing engines

When done using the device, and perhaps the module needs to be unloaded,
the driver needs to take the following steps:

  - Disable the device from generating IRQs
  - Release the IRQ (free_irq())
  - Stop all DMA activity
  - Release DMA buffers (both streaming and coherent)
  - Unregister from other subsystems (e.g. scsi or netdev)
  - Release MMIO/IOP resources
  - Disable the device

Most of these topics are covered in the following sections.
For the rest look at LDD3 or <linux/pci.h> .

If the PCI subsystem is not configured (CONFIG_PCI is not set), most of
the PCI functions described below are defined as inline functions either
completely empty or just returning an appropriate error codes to avoid
lots of ifdefs in the drivers.

pci_register_driver()와 ID table

PCI device driver는 초기화 중 `struct pci_driver`를 가리키는 pointer로 `pci_register_driver()`를 호출합니다. 구조체 문서는 `include/linux/pci.h`의 kernel-doc에서 생성됩니다.

ID table은 `struct pci_device_id` entry 배열이며 all-zero entry로 끝납니다. 일반적으로 `static const` 정의가 권장되고 구조체 문서는 `include/linux/mod_devicetable.h`에서 생성됩니다. 대부분 driver는 `PCI_DEVICE()` 또는 `PCI_DEVICE_CLASS()`만 있으면 table을 만들 수 있습니다.

Runtime에는 sysfs `new_id`에 hexadecimal field를 기록해 driver의 `pci_ids` table에 새 ID를 추가할 수 있습니다. Leading `0x`는 쓰지 않습니다.

echo "vendor device subvendor subdevice class class_mask driver_data" > \
/sys/bus/pci/drivers/{driver}/new_id

`driver_data`는 driver에 정의된 `pci_device_id` entry의 값과 일치해야 합니다. 모든 entry의 `driver_data`가 non-zero라면 이 field는 사실상 필수입니다.

ID가 추가되면 갱신된 `pci_ids` 목록의 아직 claim되지 않은 device마다 probe routine을 호출합니다. Driver 종료 시 `pci_unregister_driver()`만 호출하면 PCI layer가 driver가 처리하던 모든 device의 remove hook을 자동 호출합니다.

pci_register_driver() call
==========================

PCI device drivers call ``pci_register_driver()`` during their
initialization with a pointer to a structure describing the driver
(``struct pci_driver``):

.. kernel-doc:: include/linux/pci.h
   :functions: pci_driver

The ID table is an array of ``struct pci_device_id`` entries ending with an
all-zero entry.  Definitions with static const are generally preferred.

.. kernel-doc:: include/linux/mod_devicetable.h
   :functions: pci_device_id

Most drivers only need ``PCI_DEVICE()`` or ``PCI_DEVICE_CLASS()`` to set up
a pci_device_id table.

New PCI IDs may be added to a device driver pci_ids table at runtime
as shown below::

  echo "vendor device subvendor subdevice class class_mask driver_data" > \
  /sys/bus/pci/drivers/{driver}/new_id

All fields are passed in as hexadecimal values (no leading 0x).
The vendor and device fields are mandatory, the others are optional. Users
need pass only as many optional fields as necessary:

  - subvendor and subdevice fields default to PCI_ANY_ID (FFFFFFFF)
  - class and classmask fields default to 0
  - driver_data defaults to 0UL.
  - override_only field defaults to 0.

Note that driver_data must match the value used by any of the pci_device_id
entries defined in the driver. This makes the driver_data field mandatory
if all the pci_device_id entries have a non-zero driver_data value.

Once added, the driver probe routine will be invoked for any unclaimed
PCI devices listed in its (newly updated) pci_ids list.

When the driver exits, it just calls pci_unregister_driver() and the PCI layer
automatically calls the remove hook for all devices handled by the driver.

초기화·종료 attribute

`<linux/init.h>`의 `__init`은 driver 초기화 뒤 버리는 initialization code, `__exit`는 non-modular driver에서 무시하는 exit code를 표시합니다.

`module_init()`·`module_exit()` function과 오직 이들이 호출하는 initialization function에 `__init`·`__exit`를 붙입니다. `struct pci_driver`에는 붙이지 마십시오. 어떤 mark인지 확실하지 않다면 잘못 붙이는 것보다 붙이지 않는 편이 낫습니다.

"Attributes" for driver functions/data
--------------------------------------

Please mark the initialization and cleanup functions where appropriate
(the corresponding macros are defined in <linux/init.h>):

        ======                =================================================
        __init                Initialization code. Thrown away after the driver
                        initializes.
        __exit                Exit code. Ignored for non-modular drivers.
        ======                =================================================

Tips on when/where to use the above attributes:
        - The module_init()/module_exit() functions (and all
          initialization functions called _only_ from these)
          should be marked __init/__exit.

        - Do not mark the struct pci_driver.

        - Do NOT mark a function if you are not sure which mark to use.
          Better to not mark the function than mark the function wrong.

PCI device 수동 검색

PCI driver가 device 검색에 `pci_register_driver()`를 쓰지 않으려면 매우 타당한 이유가 있어야 합니다. 주된 이유는 하나의 PCI device가 serial·parallel port·floppy controller처럼 여러 hardware service를 구현해 여러 driver가 제어하는 경우입니다.

Vendor·device ID 검색은 `pci_get_device()`, class ID 검색은 `pci_get_class()`, vendor/device와 subsystem vendor/device를 함께 검색할 때는 `pci_get_subsys()`를 사용합니다.

struct pci_dev *dev = NULL;
while (dev = pci_get_device(VENDOR_ID, DEVICE_ID, dev))
        configure_device(dev);

pci_get_class(CLASS_ID, dev);
pci_get_subsys(VENDOR_ID, DEVICE_ID, SUBSYS_VENDOR_ID, SUBSYS_DEVICE_ID, dev);

`PCI_ANY_ID`를 `VENDOR_ID` 또는 `DEVICE_ID` wildcard로 사용할 수 있어 특정 vendor의 모든 device 등을 찾을 수 있습니다.

이 function들은 hotplug-safe이며 반환하는 `pci_dev` reference count를 증가시킵니다. 결국, 필요하면 module unload 시점에 `pci_dev_put()`으로 reference를 줄여야 합니다.

How to find PCI devices manually
================================

PCI drivers should have a really good reason for not using the
pci_register_driver() interface to search for PCI devices.
The main reason PCI devices are controlled by multiple drivers
is because one PCI device implements several different HW services.
E.g. combined serial/parallel port/floppy controller.

A manual search may be performed using the following constructs:

Searching by vendor and device ID::

        struct pci_dev *dev = NULL;
        while (dev = pci_get_device(VENDOR_ID, DEVICE_ID, dev))
                configure_device(dev);

Searching by class ID (iterate in a similar way)::

        pci_get_class(CLASS_ID, dev)

Searching by both vendor/device and subsystem vendor/device ID::

        pci_get_subsys(VENDOR_ID,DEVICE_ID, SUBSYS_VENDOR_ID, SUBSYS_DEVICE_ID, dev).

You can use the constant PCI_ANY_ID as a wildcard replacement for
VENDOR_ID or DEVICE_ID.  This allows searching for any device from a
specific vendor, for example.

These functions are hotplug-safe. They increment the reference count on
the pci_dev that they return. You must eventually (possibly at module unload)
decrement the reference count on these devices by calling pci_dev_put().

Device 초기화와 config access

대부분의 PCI driver 초기화는 device enable, MMIO/IOP 요청, streaming·coherent DMA mask, shared control data, config space, IRQ, non-PCI 부분, DMA engine 순입니다.

Driver는 거의 언제든 PCI config space register에 접근할 수 있습니다. 다만 BIST 실행 중에는 config space가 사라질 수 있고, 이때 PCI Bus Master Abort가 발생해 config read가 garbage를 반환합니다.

Device Initialization Steps
===========================

As noted in the introduction, most PCI drivers need the following steps
for device initialization:

  - Enable the device
  - Request MMIO/IOP resources
  - Set the DMA mask size (for both coherent and streaming DMA)
  - Allocate and initialize shared control data (pci_allocate_coherent())
  - Access device configuration space (if needed)
  - Register IRQ handler (request_irq())
  - Initialize non-PCI (i.e. LAN/SCSI/etc parts of the chip)
  - Enable DMA/processing engines.

The driver can access PCI config space registers at any time.
(Well, almost. When running BIST, config space can go away...but
that will just result in a PCI Bus Master Abort and config reads
will return garbage).

PCI device enable·bus master·MWI

Device register를 만지기 전에 `pci_enable_device()`를 호출해야 합니다. Suspended device를 깨우고 BIOS가 하지 않았다면 I/O·memory region과 IRQ를 할당합니다. 실패할 수 있으므로 반드시 return value를 확인하십시오.

알려진 OS bug로 resource allocation을 확인하기 전에 resource를 enable합니다. `pci_request_resources()`를 먼저 부르는 편이 논리적이지만 현재 driver는 두 device에 같은 range가 배정된 문제를 감지하지 못합니다. 흔하지 않고 곧 고쳐질 가능성도 낮습니다.

`pci_set_master()`는 `PCI_COMMAND`의 bus master bit를 설정해 DMA를 enable하고 BIOS가 잘못 지정한 latency timer도 고칩니다. `pci_clear_master()`는 bit를 지워 DMA를 disable합니다.

PCI Memory-Write-Invalidate transaction을 지원하면 `pci_set_mwi()`를 호출합니다. `PCI_COMMAND`의 Mem-Wr-Inval bit를 enable하고 cache line size register도 바로 설정합니다. Architecture나 chipset이 지원하지 않을 수 있으므로 return value를 확인합니다. 필수는 아니라면 `pci_try_set_mwi()`로 best effort를 요청합니다.

Enable the PCI device
---------------------
Before touching any device registers, the driver needs to enable
the PCI device by calling pci_enable_device(). This will:

  - wake up the device if it was in suspended state,
  - allocate I/O and memory regions of the device (if BIOS did not),
  - allocate an IRQ (if BIOS did not).

.. note::
   pci_enable_device() can fail! Check the return value.

.. warning::
   OS BUG: we don't check resource allocations before enabling those
   resources. The sequence would make more sense if we called
   pci_request_resources() before calling pci_enable_device().
   Currently, the device drivers can't detect the bug when two
   devices have been allocated the same range. This is not a common
   problem and unlikely to get fixed soon.

   This has been discussed before but not changed as of 2.6.19:
   https://lore.kernel.org/r/20060302180025.GC28895@flint.arm.linux.org.uk/


pci_set_master() will enable DMA by setting the bus master bit
in the PCI_COMMAND register. It also fixes the latency timer value if
it's set to something bogus by the BIOS.  pci_clear_master() will
disable DMA by clearing the bus master bit.

If the PCI device can use the PCI Memory-Write-Invalidate transaction,
call pci_set_mwi().  This enables the PCI_COMMAND bit for Mem-Wr-Inval
and also ensures that the cache line size register is set correctly.
Check the return value of pci_set_mwi() as not all architectures
or chip-sets may support Memory-Write-Invalidate.  Alternatively,
if Mem-Wr-Inval would be nice to have but is not required, call
pci_try_set_mwi() to have the system do its best effort at enabling
Mem-Wr-Inval.

MMIO·IOP resource 요청

MMIO memory와 I/O port address를 PCI config space에서 직접 읽지 마십시오. Architecture·chipset code가 PCI bus address를 host physical address로 remap했을 수 있으므로 `pci_dev`의 값을 사용합니다. Register와 device memory 접근법은 `Documentation/driver-api/io-mapping.rst`를 참조하십시오.

Driver는 `pci_request_region()`으로 같은 address resource를 다른 device가 사용하지 않는지 확인해야 합니다. 반대로 `pci_disable_device()` 호출 뒤 `pci_release_region()`으로 해제합니다. 두 device의 address range 충돌을 막기 위한 것입니다.

현재 driver는 `pci_enable_device()` 뒤에야 MMIO와 I/O Port resource 가용성을 판단할 수 있습니다.

Normal PCI BAR에 설명되지 않은 resource에는 MMIO용 `request_mem_region()`, I/O Port용 `request_region()`을 사용합니다. `pci_request_selected_regions()`도 참조하십시오.

Request MMIO/IOP resources
--------------------------
Memory (MMIO), and I/O port addresses should NOT be read directly
from the PCI device config space. Use the values in the pci_dev structure
as the PCI "bus address" might have been remapped to a "host physical"
address by the arch/chip-set specific kernel support.

See Documentation/driver-api/io-mapping.rst for how to access device registers
or device memory.

The device driver needs to call pci_request_region() to verify
no other device is already using the same address resource.
Conversely, drivers should call pci_release_region() AFTER
calling pci_disable_device().
The idea is to prevent two devices colliding on the same address range.

.. tip::
   See OS BUG comment above. Currently (2.6.19), The driver can only
   determine MMIO and IO Port resource availability _after_ calling
   pci_enable_device().

Generic flavors of pci_request_region() are request_mem_region()
(for MMIO ranges) and request_region() (for IO Port ranges).
Use these for address resources that are not described by "normal" PCI
BARs.

Also see pci_request_selected_regions() below.

DMA mask 설정

권위 있는 DMA interface 설명은 `Documentation/core-api/dma-api.rst`를 참조하십시오. 이 절은 driver가 device DMA capability를 표시해야 한다는 알림입니다.

모든 driver는 PCI bus master의 32-bit 또는 64-bit DMA capability를 명시해야 합니다. Streaming data에 32-bit보다 넓은 bus master capability가 있으면 적절한 parameter로 `dma_set_mask()`를 호출해 등록합니다. System RAM이 4GiB physical address 위에 있는 system에서 더 효율적인 DMA가 가능합니다.

PCI-X와 PCIe compliant device는 64-bit DMA device이므로 모든 해당 driver가 `dma_set_mask()`를 호출해야 합니다.

Device가 4GiB 위 System RAM의 coherent memory를 직접 address할 수 있으면 `dma_set_coherent_mask()`로 등록합니다. 초기 64-bit PCI device와 일부 PCI-X device는 payload streaming DMA는 64-bit지만 control coherent DMA는 그렇지 않을 수 있습니다.

Set the DMA mask size
---------------------
.. note::
   If anything below doesn't make sense, please refer to
   Documentation/core-api/dma-api.rst. This section is just a reminder that
   drivers need to indicate DMA capabilities of the device and is not
   an authoritative source for DMA interfaces.

While all drivers should explicitly indicate the DMA capability
(e.g. 32 or 64 bit) of the PCI bus master, devices with more than
32-bit bus master capability for streaming data need the driver
to "register" this capability by calling dma_set_mask() with
appropriate parameters.  In general this allows more efficient DMA
on systems where System RAM exists above 4G _physical_ address.

Drivers for all PCI-X and PCIe compliant devices must call
dma_set_mask() as they are 64-bit DMA devices.

Similarly, drivers must also "register" this capability if the device
can directly address "coherent memory" in System RAM above 4G physical
address by calling dma_set_coherent_mask().
Again, this includes drivers for all PCI-X and PCIe compliant devices.
Many 64-bit "PCI" devices (before PCI-X) and some PCI-X devices are
64-bit DMA capable for payload ("streaming") data but not control
("coherent") data.

Shared control data 설정

DMA mask를 설정한 뒤 driver는 coherent, 즉 shared memory를 할당할 수 있습니다. DMA API 전체 설명은 `Documentation/core-api/dma-api.rst`에 있습니다. Device에서 DMA를 enable하기 전에 완료해야 합니다.

Setup shared control data
-------------------------
Once the DMA masks are set, the driver can allocate "coherent" (a.k.a. shared)
memory.  See Documentation/core-api/dma-api.rst for a full description of
the DMA APIs. This section is just a reminder that it needs to be done
before enabling DMA on the device.

Device register 초기화

일부 driver는 특정 capability field를 program하거나 vendor-specific register를 초기화·reset해야 합니다. Pending interrupt를 clear하는 것이 예입니다.

Initialize device registers
---------------------------
Some drivers will need specific "capability" fields programmed
or other "vendor specific" register initialized or reset.
E.g. clearing pending interrupts.

IRQ handler와 MSI/MSI-X

`request_irq()`는 여기서 마지막 단계로 설명되지만 device 초기화의 중간 단계일 때가 많고, device를 실제 open할 때까지 미룰 수도 있습니다.

모든 PCI IRQ line을 공유할 수 있으므로 handler는 `IRQF_SHARED`로 등록하고 `devid`로 IRQ를 device에 mapping해야 합니다. `request_irq()`는 interrupt number에 handler와 device handle을 연결하고 interrupt를 enable합니다. 등록 전에 device를 quiesce하고 pending interrupt가 없어야 합니다.

전통적인 interrupt number는 PCI device에서 interrupt controller로 가는 IRQ line이지만 MSI/MSI-X에서는 CPU vector입니다.

MSI와 MSI-X는 Local APIC로 DMA write를 보내 interrupt를 전달합니다. MSI는 연속 vector block이 필요하고 MSI-X는 개별 vector 여러 개를 할당할 수 있습니다.

`request_irq()` 전에 `PCI_IRQ_MSI`·`PCI_IRQ_MSIX` flag로 `pci_alloc_irq_vectors()`를 호출합니다. 많은 architecture·chipset·BIOS가 MSI/MSI-X를 지원하지 않아 두 flag만 쓰면 실패할 수 있으므로 가능하면 `PCI_IRQ_INTX`도 지정합니다.

MSI/MSI-X와 legacy INTx handler가 다르면 할당 뒤 `pci_dev`의 `msi_enabled`와 `msix_enabled` flag로 올바른 handler를 선택합니다.

MSI는 독점 vector이므로 handler가 자기 device가 interrupt 원인인지 확인하지 않아도 됩니다. 또한 MSI 전달 시 DMA가 host CPU에 보인다는 ordering 보장으로 stale control data와 DMA/IRQ race를 피하고, DMA stream flush용 MMIO read를 생략할 수 있습니다.

사용 예는 `drivers/infiniband/hw/mthca/`와 `drivers/net/tg3.c`를 참조하십시오.

Register IRQ handler
--------------------
While calling request_irq() is the last step described here,
this is often just another intermediate step to initialize a device.
This step can often be deferred until the device is opened for use.

All interrupt handlers for IRQ lines should be registered with IRQF_SHARED
and use the devid to map IRQs to devices (remember that all PCI IRQ lines
can be shared).

request_irq() will associate an interrupt handler and device handle
with an interrupt number. Historically interrupt numbers represent
IRQ lines which run from the PCI device to the Interrupt controller.
With MSI and MSI-X (more below) the interrupt number is a CPU "vector".

request_irq() also enables the interrupt. Make sure the device is
quiesced and does not have any interrupts pending before registering
the interrupt handler.

MSI and MSI-X are PCI capabilities. Both are "Message Signaled Interrupts"
which deliver interrupts to the CPU via a DMA write to a Local APIC.
The fundamental difference between MSI and MSI-X is how multiple
"vectors" get allocated. MSI requires contiguous blocks of vectors
while MSI-X can allocate several individual ones.

MSI capability can be enabled by calling pci_alloc_irq_vectors() with the
PCI_IRQ_MSI and/or PCI_IRQ_MSIX flags before calling request_irq(). This
causes the PCI support to program CPU vector data into the PCI device
capability registers. Many architectures, chip-sets, or BIOSes do NOT
support MSI or MSI-X and a call to pci_alloc_irq_vectors with just
the PCI_IRQ_MSI and PCI_IRQ_MSIX flags will fail, so try to always
specify PCI_IRQ_INTX as well.

Drivers that have different interrupt handlers for MSI/MSI-X and
legacy INTx should chose the right one based on the msi_enabled
and msix_enabled flags in the pci_dev structure after calling
pci_alloc_irq_vectors.

There are (at least) two really good reasons for using MSI:

1) MSI is an exclusive interrupt vector by definition.
   This means the interrupt handler doesn't have to verify
   its device caused the interrupt.

2) MSI avoids DMA/IRQ race conditions. DMA to host memory is guaranteed
   to be visible to the host CPU(s) when the MSI is delivered. This
   is important for both data coherency and avoiding stale control data.
   This guarantee allows the driver to omit MMIO reads to flush
   the DMA stream.

See drivers/infiniband/hw/mthca/ or drivers/net/tg3.c for examples
of MSI/MSI-X usage.

PCI device shutdown 순서

PCI driver unload 시 device IRQ 생성 disable, `free_irq()`, 모든 DMA 정지, streaming·coherent buffer 해제, SCSI·netdev 같은 subsystem 등록 해제, MMIO/IO Port 응답 disable, resource 해제를 수행해야 합니다.

PCI device shutdown
===================

When a PCI device driver is being unloaded, most of the following
steps need to be performed:

  - Disable the device from generating IRQs
  - Release the IRQ (free_irq())
  - Stop all DMA activity
  - Release DMA buffers (both streaming and coherent)
  - Unregister from other subsystems (e.g. scsi or netdev)
  - Disable device from responding to MMIO/IO Port addresses
  - Release MMIO/IO Port resource(s)

Device IRQ 중지

IRQ 중지 방법은 chip·device별입니다. 하지 않으면 shared IRQ에서 screaming interrupt가 발생할 수 있습니다.

Shared handler를 unhook해도 다른 device 때문에 IRQ line은 enable 상태입니다. 제거된 device가 line을 assert하면 남은 device의 interrupt로 오인하지만 아무 handler도 처리하지 않아 system이 100,000회 반복 뒤 IRQ를 mask할 때까지 hang합니다. 그러면 같은 shared IRQ의 나머지 device도 정상 동작하지 않습니다.

MSI/MSI-X는 독점 interrupt로 정의되어 screaming interrupt 문제를 피할 수 있으므로 이를 사용할 또 하나의 이유입니다.

Stop IRQs on the device
-----------------------
How to do this is chip/device specific. If it's not done, it opens
the possibility of a "screaming interrupt" if (and only if)
the IRQ is shared with another device.

When the shared IRQ handler is "unhooked", the remaining devices
using the same IRQ line will still need the IRQ enabled. Thus if the
"unhooked" device asserts IRQ line, the system will respond assuming
it was one of the remaining devices asserted the IRQ line. Since none
of the other devices will handle the IRQ, the system will "hang" until
it decides the IRQ isn't going to get handled and masks the IRQ (100,000
iterations later). Once the shared IRQ is masked, the remaining devices
will stop functioning properly. Not a nice situation.

This is another reason to use MSI or MSI-X if it's available.
MSI and MSI-X are defined to be exclusive interrupts and thus
are not susceptible to the "screaming interrupt" problem.

IRQ 해제와 DMA 중지

Device가 quiesce되어 더 이상 IRQ를 만들지 않으면 `free_irq()`를 호출합니다. Pending IRQ 처리가 끝난 뒤 handler를 unhook하고 다른 사용자가 없으면 IRQ를 해제합니다.

DMA control data를 deallocate하기 전에 모든 DMA operation을 중지하는 것은 매우 중요합니다. 실패하면 memory corruption, hang, 일부 chipset의 hard crash가 발생할 수 있습니다.

IRQ를 먼저 중지한 뒤 DMA를 멈추면 handler가 DMA engine을 다시 시작하는 race를 피할 수 있습니다. 명백해 보이지만 과거 여러 성숙한 driver도 잘못 처리했습니다.

Release the IRQ
---------------
Once the device is quiesced (no more IRQs), one can call free_irq().
This function will return control once any pending IRQs are handled,
"unhook" the drivers IRQ handler from that IRQ, and finally release
the IRQ if no one else is using it.


Stop all DMA activity
---------------------
It's extremely important to stop all DMA operations BEFORE attempting
to deallocate DMA control data. Failure to do so can result in memory
corruption, hangs, and on some chip-sets a hard crash.

Stopping DMA after stopping the IRQs can avoid races where the
IRQ handler might restart DMA engines.

While this step sounds obvious and trivial, several "mature" drivers
didn't get this step right in the past.

DMA buffer와 subsystem 정리

DMA를 멈춘 뒤 streaming DMA부터 정리합니다. Data buffer를 unmap하고 upstream owner가 있으면 반환합니다. 그 다음 control data가 든 coherent buffer를 정리합니다. Unmap interface는 `Documentation/core-api/dma-api.rst`를 참조하십시오.

대부분 low-level PCI driver는 USB, ALSA, SCSI, NetDev, Infiniband 같은 다른 subsystem을 지원합니다. 해당 subsystem resource를 빠뜨리지 마십시오. 빠뜨리면 unload된 driver를 subsystem이 호출할 때 Oops 또는 panic이 나는 경우가 많습니다.

Release DMA buffers
-------------------
Once DMA is stopped, clean up streaming DMA first.
I.e. unmap data buffers and return buffers to "upstream"
owners if there is one.

Then clean up "coherent" buffers which contain the control data.

See Documentation/core-api/dma-api.rst for details on unmapping interfaces.


Unregister from other subsystems
--------------------------------
Most low level PCI device drivers support some other subsystem
like USB, ALSA, SCSI, NetDev, Infiniband, etc. Make sure your
driver isn't losing resources from that other subsystem.
If this happens, typically the symptom is an Oops (panic) when
the subsystem attempts to call into a driver that has been unloaded.

Device disable과 region 해제

MMIO 또는 I/O Port resource를 `io_unmap()`한 뒤 `pci_disable_device()`를 호출합니다. 이는 `pci_enable_device()`의 대칭 반대이며 이후 device register에 접근하면 안 됩니다.

`pci_release_region()`으로 MMIO 또는 I/O Port range를 사용 가능 상태로 표시합니다. 하지 않으면 보통 driver를 다시 load할 수 없습니다.

Disable Device from responding to MMIO/IO Port addresses
--------------------------------------------------------
io_unmap() MMIO or IO Port resources and then call pci_disable_device().
This is the symmetric opposite of pci_enable_device().
Do not access device registers after calling pci_disable_device().


Release MMIO/IO Port Resource(s)
--------------------------------
Call pci_release_region() to mark the MMIO or IO Port range as available.
Failure to do so usually results in the inability to reload the driver.

PCI config space 접근

`struct pci_dev *`가 있으면 `pci_(read|write)_config_(byte|word|dword)`로 config space에 접근합니다. 성공 시 0, 실패 시 `PCIBIOS_...` error code를 반환하며 `pcibios_strerror`로 문자열로 바꿀 수 있습니다. 대개 driver는 유효한 PCI device 접근이 실패하지 않는다고 봅니다.

`pci_dev`가 없으면 `pci_bus_(read|write)_config_(byte|word|dword)`로 해당 bus의 device와 function에 접근합니다.

Standard config header field에는 `<linux/pci.h>`의 location·bit symbolic name을 사용하십시오. Extended PCI Capability register는 capability별 `pci_find_capability()`로 해당 register block을 찾습니다.

How to access PCI config space
==============================

You can use `pci_(read|write)_config_(byte|word|dword)` to access the config
space of a device represented by `struct pci_dev *`. All these functions return
0 when successful or an error code (`PCIBIOS_...`) which can be translated to a
text string by pcibios_strerror. Most drivers expect that accesses to valid PCI
devices don't fail.

If you don't have a struct pci_dev available, you can call
`pci_bus_(read|write)_config_(byte|word|dword)` to access a given device
and function on that bus.

If you access fields in the standard portion of the config header, please
use symbolic names of locations and bits declared in <linux/pci.h>.

If you need to access Extended PCI Capability registers, just call
pci_find_capability() for the particular capability and it will find the
corresponding register block for you.

기타 유용한 function

Other interesting functions
===========================

=============================        ================================================
pci_get_domain_bus_and_slot()        Find pci_dev corresponding to given domain,
                                bus and slot and number. If the device is
                                found, its reference count is increased.
pci_set_power_state()                Set PCI Power Management state (0=D0 ... 3=D3)
pci_find_capability()                Find specified capability in device's capability
                                list.
pci_resource_start()                Returns bus start address for a given PCI region
pci_resource_end()                Returns bus end address for a given PCI region
pci_resource_len()                Returns the byte length of a PCI region
pci_set_drvdata()                Set private driver data pointer for a pci_dev
pci_get_drvdata()                Return private driver data pointer for a pci_dev
pci_set_mwi()                        Enable Memory-Write-Invalidate transactions.
pci_clear_mwi()                        Disable Memory-Write-Invalidate transactions.
=============================        ================================================

기타 힌트

사용자에게 PCI device 이름을 표시할 때는 `pci_name(pci_dev)`를 사용하십시오.

PCI device는 항상 `pci_dev` structure pointer로 참조하십시오. 모든 PCI layer function이 이 식별을 사용하며 유일하게 합리적입니다. Multi-primary-bus system에서는 의미가 복잡하므로 특별한 목적 외에는 bus/slot/function number를 쓰지 마십시오.

Driver에서 Fast Back to Back write를 켜지 마십시오. Bus의 모든 device가 지원해야 하므로 개별 driver가 아니라 platform과 generic code가 처리해야 합니다.

Miscellaneous hints
===================

When displaying PCI device names to the user (for example when a driver wants
to tell the user what card has it found), please use pci_name(pci_dev).

Always refer to the PCI devices by a pointer to the pci_dev structure.
All PCI layer functions use this identification and it's the only
reasonable one. Don't use bus/slot/function numbers except for very
special purposes -- on systems with multiple primary buses their semantics
can be pretty complex.

Don't try to turn on Fast Back to Back writes in your driver.  All devices
on the bus need to be capable of doing it, so this is something which needs
to be handled by platform and generic code, not individual drivers.

Vendor·device ID

여러 driver가 공유하지 않는 새 device 또는 vendor ID를 `include/linux/pci_ids.h`에 추가하지 마십시오. 유용하면 driver 내부 private definition을 만들거나 plain hexadecimal constant를 사용합니다.

Device ID는 vendor가 정하는 임의 hexadecimal number이며 보통 `pci_device_id` table 한 곳에서만 사용합니다.

새 vendor/device ID는 `https://pci-ids.ucw.cz/`에 제출하십시오. `pci.ids` mirror는 `https://github.com/pciutils/pciids`에 있습니다.

Vendor and device identifications
=================================

Do not add new device or vendor IDs to include/linux/pci_ids.h unless they
are shared across multiple drivers.  You can add private definitions in
your driver if they're helpful, or just use plain hex constants.

The device IDs are arbitrary hex numbers (vendor controlled) and normally used
only in a single location, the pci_device_id table.

Please DO submit new vendor/device IDs to https://pci-ids.ucw.cz/.
There's a mirror of the pci.ids file at https://github.com/pciutils/pciids.

폐기된 function

오래된 driver를 새 PCI interface로 port할 때 만날 수 있는 아래 function은 hotplug, PCI domain, 올바른 locking과 호환되지 않아 kernel에서 제거됐습니다.

전통적으로 PCI device list를 직접 순회하는 driver도 여전히 가능하지만 권장하지 않습니다.

Obsolete functions
==================

There are several functions which you might come across when trying to
port an old driver to the new PCI interface.  They are no longer present
in the kernel as they aren't compatible with hotplug or PCI domains or
having sane locking.

=================        ===========================================
pci_find_device()        Superseded by pci_get_device()
pci_find_subsys()        Superseded by pci_get_subsys()
pci_find_slot()                Superseded by pci_get_domain_bus_and_slot()
pci_get_slot()                Superseded by pci_get_domain_bus_and_slot()
=================        ===========================================

The alternative is the traditional PCI device driver that walks PCI
device lists. This is still possible but discouraged.

MMIO와 write posting

Driver를 I/O Port space에서 MMIO space로 바꾸면 write posting을 처리해야 합니다. `tg3`, `acenic`, `sym53c8xx_2` 등 많은 driver가 이미 처리합니다.

I/O Port write는 CPU가 계속 실행하기 전에 transaction이 PCI device에 도착함을 보장합니다. MMIO write는 도착 전에 CPU가 계속할 수 있습니다. Transaction이 destination에 닿기 전에 write completion을 CPU에 post하기 때문에 write posting이라고 합니다.

따라서 timing-sensitive code에서 CPU가 기다려야 하는 지점에는 `readl()` 같은 read를 추가해야 합니다. I/O Port bit-banging은 write 후 delay만으로 충분합니다.

for (i = 8; --i; val >>= 1) {
        outb(val & 1, ioport_reg);      /* write bit */
        udelay(10);
}

MMIO에서는 write 뒤 안전한 register를 read해 posted write를 flush한 다음 delay합니다.

for (i = 8; --i; val >>= 1) {
        writeb(val & 1, mmio_reg);      /* write bit */
        readb(safe_mmio_reg);           /* flush posted write */
        udelay(10);
}

`safe_mmio_reg`의 read에는 device 정상 동작을 방해하는 side effect가 없어야 합니다.

MMIO Space and "Write Posting"
==============================

Converting a driver from using I/O Port space to using MMIO space
often requires some additional changes. Specifically, "write posting"
needs to be handled. Many drivers (e.g. tg3, acenic, sym53c8xx_2)
already do this. I/O Port space guarantees write transactions reach the PCI
device before the CPU can continue. Writes to MMIO space allow the CPU
to continue before the transaction reaches the PCI device. HW weenies
call this "Write Posting" because the write completion is "posted" to
the CPU before the transaction has reached its destination.

Thus, timing sensitive code should add readl() where the CPU is
expected to wait before doing other work.  The classic "bit banging"
sequence works fine for I/O Port space::

       for (i = 8; --i; val >>= 1) {
               outb(val & 1, ioport_reg);      /* write bit */
               udelay(10);
       }

The same sequence for MMIO space should be::

       for (i = 8; --i; val >>= 1) {
               writeb(val & 1, mmio_reg);      /* write bit */
               readb(safe_mmio_reg);           /* flush posted write */
               udelay(10);
       }

It is important that "safe_mmio_reg" not have any side effects that
interferes with the correct operation of the device.

PCI reset 시 write flush

PCI device reset에서도 주의해야 합니다. `writel()`을 flush할 때 PCI Configuration space read를 사용하면 device가 `readl()`에 응답하지 않아 발생하는 PCI master abort를 모든 platform에서 안전하게 처리할 수 있습니다.

대부분 x86 platform은 MMIO read master abort를 soft fail로 처리해 `~0` 같은 garbage를 반환하지만, 많은 RISC platform에서는 hard fail로 crash합니다.

Another case to watch out for is when resetting a PCI device. Use PCI
Configuration space reads to flush the writel(). This will gracefully
handle the PCI master abort on all platforms if the PCI device is
expected to not respond to a readl().  Most x86 platforms will allow
MMIO reads to master abort (a.k.a. "Soft Fail") and return garbage
(e.g. ~0). But many RISC platforms will crash (a.k.a."Hard Fail").